A typical planar heating assembly is composed of a heating element of metallic foil and a pair of sheets made of plastic material which are equal to each other in shape and size. In assembly, the heating element is provided on one surface of one sheet and then the other sheet is bonded to that surface so that the heating element is clamped between the sheets.
In many cases, the heating assembly is required to be wholly enclosed by protective means so as to be airtight and electrically insulated. Typically, such protective means consists of a pair of protective covers made of insulating sheet material which are substantially larger in size than the heating assembly to be enclosed by them. The protective covers are positioned on both surfaces of the heating assembly in such a manner that peripheral areas of the respective protective covers extend beyond a margin of the heating assembly, and then the protective covers are bonded together by fusion along their opposed peripheral areas to form a planar heat generator which is airtight and electrically insulated.
If any air is trapped between the heating assembly and the protective covers, heat from the heating element when in use would cause an excessive increase in temperature in the local areas where air bubbles are contained. To avoid this situation, it is necessary to ensure a state of close adherence between the protective covers and both surfaces of the heating assembly. However, the plastic sheets composing the heating assembly are usually made of polyester, while the protective covers are usually made of polyvinyl chloride, and since these respective plastic materials differ in melting point from each other, great difficulty has been encountered in providing for perfect adherence between them by fusion.
Some efforts have been made toward solving the abovementioned problem. One solution is disclosed in Japanese Utility Model Public Disclosure No. 58-152794 in which the protective covers are directly bonded to each other by fusion through plural holes which are bored through the heating assembly at portions other than those where the heating element is located.
However, the planar heat generator disclosed in the above Japanese U.M. Public Disclosure has been found to be unacceptable because of the following disadvantages. First, the outer surfaces of the planar heat generator thus obtained are not smooth and have recesses thereon because of the existence of the holes under the protective covers. This not only prevents successful application of embossing or screen printing thereon but also impairs the appearance of the finished product. In addition, some difficulties are involved in labelling thereon. Secondly, a serious problem is caused in the fusion process. It is necessary in a fusion process to apply uniform pressure on the protective covers in order to expel the air between the covers and the heating assembly. However, some holes are in fact less firmly pressed than the rest and, accordingly, air bubbles are left in these holes. In a case where heat is supplied during the thermal adhesion of metallic foil shielding layers on both surfaces of the heat generator at a subsequent stage, these air bubbles are expanded by the heat. Even after cooling, the resulting expansion is not eliminated and undesirable irregularity remains on each shielding layer surface. This irregularity also prevents successful application of the above-mentioned embossing, screen printing or labelling and impairs the appearance of the finished product.